Various recirculation systems have been developed in an effort to reduce the amount of water that is wasted by users of conventional plumbing systems while waiting for hot water to arrive at their tap. Most users just let the hot water faucet run until the cooled hot water has been removed from their pipes, and hot water arrives at the faucet. This method, however, wastes a lot of water down the drain. Some people have developed recirculation systems that constantly pump the cooled hot water into the cold water line. This water is returned to the water heater.
For example, U.S. Pat. No. 5,009,572 to Imhoff discloses a self-contained water conservation device which is installed under the sink in existing plumbing systems. The Imhoff device includes a temperature sensor that detects water temperature in the hot water line and begins pumping water into the cold water line if the water temperature in the hot water line falls below a certain preset level. However, this system pumps a significant amount of heated water into the cold water line. In the Imhoff system, a temperature sensor activates a pump that moves water from the hot water line to the cold water line. The temperature sensor activates the pump as long as the temperature in the hot water line is below a set threshold. Because the pump moves a tremendous quantity of warm water into the cold water supply, the cold water tends to become heated. This is not satisfactory for most users because their cold water ends up becoming lukewarm by the recirculation system.
Another system that has been used to provide instant hot water at a tap is disclosed in U.S. Pat. No. 5,277,219 to Lund. In this system a pump is positioned under a sink and between the hot and cold water delivery lines. Thus, the Lund system also recirculates water from the hot water delivery line through the cold water delivery line and into the hot water source. The Lund system relies on sensors to determine when the hot water faucet has been opened by a user. At this time, the pump is activated and cooled water is removed from the hot water line. In addition, Lund discusses positioning a switch at each faucet so that a user can push a button and start the pump. The pump then runs until it senses that the water in the hot water line has reached a certain temperature. However, this system also pumps a lot of warm water (or hot) into the cold water supply before the hot water temperature is sufficient to cause the sensor to turn off the pump. Thus, this system also has the same problems as the Imhoff patent discussed above.
One additional problem with recirculation systems that have been developed in the past is that their temperature sensors are not very accurate. Most systems use a temperature sensor that mounts to a water pipe and relies on the water heating the pipe to activate the sensor. This system leads to inaccurate measurements of the water temperature, thus adding to the amount of water pumped into the cold water supply. It would therefore be an advantage to provide a system that has a more sensitive mechanism for measuring the water temperature. There is a longstanding need in the technology for a means of effectively limiting the amount of warm water that enters the cold water line prior to and after the preselected water temperature is reached.